The field of the invention generally relates to the optical transmission of information and more particularly refers to the coupling of optical information to and from optical transmission fiber.
In greater particularity, the optical transmission fiber is a continuously extending single-mode fiber functioning as a duplex transmission system and the apparatus and the method of the invention concerns the selective evanescent field coupling to and from the continuous single-mode fiber at one discrete wavelength which does not interfere or interrupt the data transmission at the other discrete wavelength.
The information transfer capabilities of single-mode fiber data links are currently limited, not by the bandwidth of the fiber itself, but rather, by the modulation speed limitations of the wide variety of semiconductor laser sources and the coupling efficiencies of contemporary coupling devices. To more completely utilize the fiber bandwidth capabilities of single-mode fibers, designers have attempted a variety of wavelength multiplexing techniques which it has been hoped will permit the fabrication and realization of multigiga bit optical channels.
Typically, the duplex single-mode link depicted as prior art in FIG. 1 is relied upon. A wavelength .lambda..sub.1 from a laser source S.sub.1 passes through a coupler into the fiber and is selectively coupled out and detected by a detector D.sub.1. Optical information is transmitted in the opposite direction at a wavelength .lambda..sub.2 coming from a source S.sub.2 and detected at a detector D.sub.2. In this type of a transmission system, the two couplers are the critical components in this system in that their characteristics, including coupling loss and cross talk, determine the link's capacity and transmission length capability. Conventional bulk dichroic beamsplitters and coupling optics have been found to be unsuitable for use with diode lasers and single-mode fibers due to the couplers' lack of coupling stability and high throughput losses.
Thus, there is a continuing need in the state-of-the-art for a coupler that assures wavelength selectability in a duplex optical data transmission system and that allows a more complete utilization of the available bandwidth of a single-mode fiber data link without degrading the optical information content.